1. Field of the Invention
The instant invention relates to a cement additive containing a high glass transition temperature (Tg) polymer available as a liquid or as a redispersible powder for use in polymer-modified hydraulic portland cement mortar and concrete, the polymer-modified hydraulic portland cement mortar and concrete made therewith, and a ready-to-use portland cement composition, as well as processes used to produce the cement additive, the redispersible powder and polymer-modified hydraulic portland cement mortar and concrete.
2. Description of Related Art
Latex has been used in cementitious mixtures since the 1920s when natural rubber latex was used to improve the flexibility and adhesion of various floor topping materials. Since that time many synthetic polymers have been developed for use in portland cement, such as styrene-butadiene, polyvinyl acetate, and a variety of acrylics.
The properties that each contributes to the hardened portland cement mixture vary with the properties of each polymer type, but all have a requirement that a stabilizer, usually a surfactant, be added to prevent coagulation of the latexes in the presence of the portland cement. The prior art latexes which have previously been used as cement additives typically have a glass transition temperature below 30.degree. C. so that they form a film within the matrix of the hardened portland cement mixture. In the hardened state the film formed from the latex of the cement additive increases the tensile strength and the bond strength, and reduces the permeability of the portland cement composition.
It has been reported that in the wet state and the early stages of the hardened state of the portland cement mortar and concrete, the film formed from the latex of the cement additive impedes the passage of water vapor and maintains the high humidity conditions for the portland cement particles to cure properly. Therefore, the normal cure schedule for latex-modified cement compositions is one day of wet cure, with the remaining time exposed to air. This is in contrast to conventional, unmodified portland cement compositions which typically require seven days minimum of wet cure.
One of the drawbacks of latexes and other polymers that are film-forming at room temperature is that the mortar and concrete made with them is very difficult to clean from equipment if allowed to dry in air, even for a short time. In addition, when the surface of the wet mortar or concrete made with these latexes is exposed to air, a crust begins to form on the surface, which makes troweling and finishing difficult. These limitations not only reduce the time available for finishing, but also require that a special mixing machine, such as a mobile-mixer, be used when large quantities of latex-modified concrete are prepared in the field.
Most of the latexes used for modifying portland cement are not suitable for making into a dry, redispersible powder form. Because it has previously been assumed that film formation is a necessary requirement of a latex for it to provide the above-mentioned advantages when used as a cement additive in portland cement, latexes with a Tg of 30.degree. C. or below have typically been used for this application. However, these latexes tend to be soft and clump together in the dry powder state, and, therefore, they require added agents to aid their redispersion in water. Such redispersion agents can degrade the final properties of the mortar and concrete.
In addition, additives are needed to prevent the low Tg latex particles from forming a film during the drying process to form a redispersible powder latex. Additives for the redispersible powder latex such as polyvinyl alcohol make the hardened mortar or concrete produced with these prior art additive-containing redispersible powder latexes susceptible to deterioration when exposed to moisture. This limits the use of the presently available redispersible powder latexes to those applications where moisture will not be present.
It would be desirable to have a cement additive containing a high Tg polymer that provides the advantageous properties of polymer modification when used in hydraulic portland cement or mortar without the prior art disadvantages mentioned above. Especially desirable would be a redispersible powdered polymer for use in a cement additive which does not have the disadvantages of the prior art low Tg redispersible powders.